The present invention pertains to a projection exposure apparatus and method which may be employed, for example, where photolithographic techniques are used for manufacture of semiconductor integrated circuits, charge coupled devices and other such image pickup elements, liquid crystal display devices, thin-film magnetic heads, and other such microdevices, and pertains as well to a projection optical system suitable for use in such a projection exposure apparatus or method. The present invention permits a projection optical system to be provided which is capable of high-resolution projection of a highly detailed pattern while permitting satisfactory correction of chromatic aberration and without incurring inordinate increase in cost. Furthermore, the present invention permits a projection exposure apparatus and a projection exposure method to be provided which permit satisfactory transfer of an image of an extremely detailed pattern from a mask to a substrate.
More particularly, one or more embodiments of the present invention employ a combination of techniques for facilitating correction of chromatic aberration in the context of a projection optical system comprising one or more refractive optical members collectively comprising two or more fluoride substances. Still more particularly, one or more embodiments of the present invention utilize one or more design conditions pertaining to an illumination optical system (including light source) and/or a projection optical system for economical and/or effective allocation of such fluoride substances.
One application of projection exposure apparatuses is in photolithographic operations for manufacture of semiconductor elements or the like, where an image of a pattern on a mask, reticle, or the like (hereinafter referred to collectively as xe2x80x9cmask,xe2x80x9d xe2x80x9creticle,xe2x80x9d etc., these terms being used interchangeably where not otherwise specified) is projected by way of a projection optical system to expose resist or other such photosensitive material on a wafer, glass or other such plate, substrate, workpiece, or the like (hereinafter referred to collectively as xe2x80x9csubstrate,xe2x80x9d xe2x80x9cwafer,xe2x80x9d xe2x80x9cworkpiece,xe2x80x9d etc., these terms being used interchangeably where not otherwise specified). Accompanying the desire to achieve increased circuit density of semiconductor elements and the like, higher and higher resolutions are being required of the projection optical systems used in projection exposure apparatuses.
Resolution of an optical system is in general determined by Rayleigh""s equation, or       R    =          k      xc3x97              λ                  N          ⁢                      xe2x80x83                    ⁢          A                      ,
where xcex is the exposing wavelength, NA is the image-side numerical aperture of the projection optical system, and k is a constant which is in this case determined by resist resolution and so forth. It is clear from the above equation that resolution can be increased by decreasing the wavelength of the actinic light or radiation responsible for exposure (xe2x80x9clightxe2x80x9d and xe2x80x9cradiationxe2x80x9d are used interchangeably herein and without intention to limit either to wavelengths which are visible or invisible or the like; xe2x80x9cactinicxe2x80x9d light or radiation as used herein refers to light or radiation used for exposure without regard to whether such exposure occurs by a chemical, physical, or other process; xe2x80x9cexposurexe2x80x9d as used herein refers to any change due to receipt of such actinic light or radiation at the wafer or other such substrate or workpiece) or by increasing numerical aperture. Based on this fact, the mercury lamp i-line light sources (wavelength 365 nm) which had previously been favored by the industry have been largely replaced by the KrF excimer laser (wavelength 248 nm), and the still-shorter-wavelength ArF laser (wavelength 193 nm) is well on its way to practical application. In addition, with the goal of even further reduction in the wavelength of the light used for exposure, attempts are underway to develop an exposure apparatus utilizing an F2 laser (wavelength 157 nm).
However, increasing the numerical aperture of a projection optical system decreases it depth of focus. This in turn places stringent demands on the projection optical system with respect to correction of chromatic aberration. Furthermore, as a result of developments in resist and other peripheral technologies, the magnitude of k in the above equation has grown smaller over time. Minor aberrations and small errors in exposure dose can therefore have a large effect on resolution, and chromatic aberration must be even more tightly controlled.
One strategy which has been proposed for accomplishing such goals is the use of an exposure apparatus employing actinic radiation having a narrowed linewidth. However, where the refractive optical members in the projection optical systems of such proposed exposure apparatuses are formed from a single substance there will be a limit as to how far chromatic aberration can be corrected, making such apparatuses incapable of providing the resolutions now in demand. Furthermore, achieving narrowed linewidth is not an easy matter, and it is only with great difficulty that narrowing sufficient to permit reduction of chromatic aberration to the desired level can be achieved in the context of a projection optical system employing refractive optical members composed of a single substance. There is therefore a need for a projection optical system having optical members formed from a plurality of substances to permit further improvement in ability to correct for chromatic aberration.
However, with a light source employing an F2 laser, there are only a limited number of materials which are effective in reducing chromatic aberration, permit achievement of satisfactory transmittance, and do not present significant problems with respect to fabrication and endurance. At present, one set of materials that satisfies all of the above requirements is a combination of calcium fluoride and barium fluoride. However, barium fluoride has high specific gravity and does not lend itself to fabrication into parts having good homogeneity, and its high solvability with respect to water makes it less than suitable for fabrication. In light of the foregoing, there has been a problem in that any increase in the amount of barium fluoride in an attempt to correct chromatic aberration would lead to increased cost.
The present invention pertains to a projection exposure apparatus and method which may be employed, for example, where photolithographic techniques are used for manufacture of semiconductor integrated circuits, charge coupled devices and other such image pickup elements, liquid crystal display devices, thin-film magnetic heads, and other such microdevices, and pertains as well to a projection optical system suitable for use in such a projection exposure apparatus or method. The present invention permits a projection optical system to be provided which is capable of high-resolution projection of a highly detailed pattern while permitting satisfactory correction of chromatic aberration and without incurring inordinate increase in cost. Furthermore, the present invention permits a projection exposure apparatus and a projection exposure method to be provided which permit satisfactory transfer of an image of an extremely detailed pattern from a mask to a substrate.
More particularly, one or more embodiments of the present invention employ a combination of techniques for facilitating correction of chromatic aberration in the context of a projection optical system comprising one or more refractive optical members collectively comprising two or more fluoride substances. Still more particularly, one or more embodiments of the present invention utilize one or more design conditions pertaining to an illumination optical system (including light source) and/or a projection optical system for economical and/or effective allocation of such fluoride substances.
In order to solve one or more of the foregoing or related problems, a projection optical system associated with one aspect of the present invention is capable of projecting to an image space an image of an object in an object space, the system comprising at least one first refractive optical member comprising a first fluoride substance and at least one second refractive optical member comprising a second fluoride substance, wherein MX1 is greater than MX2 and the design condition   0.4   less than             M      ⁢              xe2x80x83            ⁢              X        2                    M      ⁢              xe2x80x83            ⁢              X        1               less than   0.87
is satisfied, where MX1 is the effective aperture of the surface or surfaces having the largest effective aperture among the surface or surfaces of the first refractive optical member or members, and MX2 is the effective aperture of the surface or surfaces having the largest effective aperture among the surface or surfaces of the second refractive optical member or members.
It is preferred in the present invention that the first fluoride substance be calcium fluoride, and that the second fluoride substance be barium fluoride. Furthermore, it is preferred in the present invention that the projection optical system furthermore comprise at least one positive lens component and at least one negative lens component, and that at least one of the positive lens component or components comprise the first fluoride substance, and that at least one of the negative lens component or components comprise the second fluoride substance. Moreover, it is preferred in the present invention that each of the lens components of the projection optical system respectively consist of only the first fluoride substance or the second fluoride substance or both. In addition, it is preferred in the present invention that the f-number of the second refractive optical member or the respective f-numbers of each of the second refractive optical members satisfy the design condition 0.8 less than |FNi|, where FNi represents each such f-number.
In order to solve one or more of the foregoing or related problems, a projection exposure apparatus associated with another aspect of the present invention is capable of transferring onto a substrate an image of a pattern on a mask, the apparatus comprising a light source capable of supplying radiation for exposure, an illumination optical system arranged to receive at least some of the radiation from the light source and guide at least some of the received radiation to the mask, and a projection optical system as described above, wherein the mask is capable of being disposed in the object space, and the substrate is capable of being disposed in the image space.
It is preferred in the present invention that such a projection exposure apparatus be capable of transferring onto a substrate an image of a pattern on a mask, the apparatus comprising a light source capable of supplying radiation for exposure, an illumination optical system arranged to receive at least some of the radiation from the light source and guide at least some of the received radiation to the mask, and a projection optical system capable of forming on the substrate an image of the pattern on the mask in correspondence to radiation received from the mask, and that the projection optical system comprise one or more refractive optical members collectively comprising at least two fluoride substances, and that a linewidth of the radiation from the light source be narrower than a natural linewidth thereof.
Furthermore, it is preferred in such a projection exposure apparatus associated with the present invention that each of the refractive optical members within such projection optical system respectively comprise one or more fluoride substances. Moreover, it is preferred in such a projection exposure apparatus associated with the present invention that the at least two fluoride substances collectively include calcium fluoride and barium fluoride.
In addition, it is preferred in such a projection exposure apparatus associated with the present invention that the at least two fluoride substances collectively include a first fluoride substance and a second fluoride substance which are such that MX1 is greater than MX2, and the design condition   0.4   less than             M      ⁢              xe2x80x83            ⁢              X        2                    M      ⁢              xe2x80x83            ⁢              X        1               less than   0.87
is satisfied, where MX1 is the effective aperture of the surface or surfaces having the largest effective aperture among the surface or surfaces of the refractive optical member or members comprising the first fluoride substance, and MX2 is the effective aperture of the surface or surfaces having the largest effective aperture among the surface or surfaces of the refractive optical member or members comprising the second fluoride substance. Furthermore, it is preferred in such a projection exposure apparatus associated with the present invention that the projection optical system furthermore comprise at least one positive lens component and at least one negative lens component, and that at least one of the positive lens component or components comprise the first fluoride substance, and that at least one of the negative lens component or components comprise the second fluoride substance.
Moreover, it is preferred in such a projection exposure apparatus associated with the present invention that the at least two fluoride substances collectively include a first fluoride substance and a second fluoride substance, and that the f-number or the respective f-numbers of the refractive optical member or members comprising the second fluoride substance satisfy the design condition 0.8 less than |FNi|, where FNi represents each such f-number.
In addition, it is preferred in such a projection exposure apparatus associated with the present invention that a linewidth of the radiation from the light source be not more than about half of a natural linewidth thereof as measured on a full-width-at-half-maximum basis. Furthermore, it is preferred in such a projection exposure apparatus associated with the present invention that the light source comprise an F2 laser. Moreover, it is preferred in such a projection exposure apparatus associated with the present invention that the light source comprise an oscillator capable of generating radiation having a linewidth narrower than a natural linewidth thereof, and an amplifier capable of amplifying the output of the radiation generated by the oscillator. In addition, it is preferred in such a projection exposure apparatus associated with the present invention that a linewidth of the radiation supplied by the light source be not more than about 0.3 pm as measured on a full-width-at-half-maximum basis. Furthermore, it is still more preferred in such a projection exposure apparatus associated with the present invention that a linewidth of the radiation supplied by the light source be not more than about 0.2 pm as measured on a full-width-at-half-maximum basis.
Moreover, it is preferred in such a projection exposure apparatus associated with the present invention that the at least two fluoride substances collectively include two species selected from among the group consisting of calcium fluoride, barium fluoride, lithium fluoride, magnesium fluoride, strontium fluoride, lithium calcium aluminum fluoride, and lithium strontium aluminum fluoride.
In order to solve one or more of the foregoing or related problems, a projection exposure method associated with another aspect of the present invention is a method for transferring onto a substrate an image of a pattern on a mask, the method using a projection exposure apparatus as described above to form on the substrate an image of the pattern on the mask.